In an associative data-processing system, the several processing units have their own operating cycles within which they examine data fed in by associated peripheral equipment to check for the occurrence of certain characteristics for which they are programmed. In its search for a given combination of such characteristics, the system must correlate the operations of these individual units or miniprocessors by the selective exchange of data among them, such exchange being initiated from time to time by one of the miniprocessors which may or may not be one of the processing units between which data are to be transferred. In a complex system, e.g. one used for traffic control in a large metropolitan area, the system is conveniently subdivided into a plurality of subsystems each including a number of miniprocessors which must, on occasion, communicate not only with one another but also with their counterparts in other subsystems.
In contrast to serially operating processing systems, whose performance is relatively time-consuming, those operating in parallel can carry out their tasks in reasonably short working periods but conventionally require cumbersone and expensive switching arrangements for their correlation.